The goal of this work is to establish a scientific basis for treating stiff-knee gait in persons with cerebral palsy. Stiff-knee gait is a prevalent, debilitating movement abnormality that is characterized by inadequate knee flexion during the swing phase. The reputed cause of stiff-knee gait is over-activity of the rectus femoris muscle; hence, individuals with stiff-knee gait frequently undergo rectus femoris transfer surgery. Unfortunately the outcomes of these procedures are inconsistent. The PIs hypothesize that several factors, in addition to over-activity of the rectus femoris, contribute to the diminished knee flexion that is commonly observed. If these factors were identified, and if methods were developed to determine which of these factors contribute to an individual's abnormal gait, then treatments for stiff-knee gait could be designed more effectively. [unreadable] [unreadable] Muscle-actuated, forward dynamic simulations will be created that reveal the causes of diminished knee flexion in subjects with stiff-knee gait and explain the functional consequences of rectus femoris transfer surgery. Aim 1 will identify the factors that can cause stiff-knee gait, and will generate an algorithm to uncover which of these factors contribute to diminished knee flexion in patients with a range of neuromusculoskeletal impairments. Aim 2 will determine why some individuals walk with diminished swing-phase knee flexion following surgeries performed to correct other aspects of their abnormal gait. Aim 3 will examine the potential of the rectus femoris to augment knee flexion after tendon transfer surgery. Aim 4 will test the validity and utility of the simulation-based analyses for guiding treatment decisions by determining whether patients have better outcomes when interventions are performed that address the biomechanical causes of their stiff-knee gait, as determined using the simulations. This work will provide new guidelines deciding which patients should undergo rectus femoris transfer surgery, and which patients are likely to benefit more from other treatments, such as bracing, strengthening exercises, injections of neuromuscular blocking agents, or other surgeries. [unreadable] [unreadable] The success of this project will result in the first (rigorously tested) simulation-based system to aid treatment planning for stiff-knee gait, and will hopefully result in better, more predictable surgical outcomes. Although multi-joint movement abnormalities such as stiff-knee gait are exceptionally complex, the development of dynamic simulations that elucidate the causes of patients' movement abnormalities is an important and necessary next step toward designing more effective treatments. [unreadable] [unreadable]